1. Field of the Invention
The invention relates to a mechanism for regulating the ink pressure within an ink reservoir of an inkjet pen. The mechanism automatically regulates the underpressure inside the inkjet pen to prevent the ink from leaking.
2. Related Art
Common inkjet printers apply ink pens that include ink reservoirs and print heads. The print head controls ink drops jetting from the ink reservoir. Two common methods for inkjet control are the thermal bubble system and the piezoelectric system. Although conventional print heads are effective for jetting ink drops from pen reservoirs, they need extra mechanisms for preventing ink from leaking out of the print heads when the print heads are inactive. These mechanisms generally provide slight underpressure at the print heads to prevent ink leakage from the pens whenever the print heads are inactive. The term xe2x80x9cunderpressurexe2x80x9d used herein means a partial vacuum (less pressure than the ambient air) within the pen reservoir that prevents flow of ink through the print head. The underpressure at the print head must be strong enough at all times for preventing ink leakage. However, the underpressure must not be so strong that the print head is unable to overcome the underpressure to jet ink drops, the size of the ink drops and the print quality are influenced, or the printing totally fails.
In order to maintain normal functions, the underpressure at the print head must be regulated within an operating range. In other words, the pressure in the ink reservoir must always be lower than the ambient pressure to prevent ink leakage, but not be too low to hinder the inkjet function. To fulfill the aforesaid requirements, many prior methods have been developed. For example, U.S. Pat. No. 4,992,802, xe2x80x9cMethod and apparatus for extending the environmental operating range of an ink jet print cartridgexe2x80x9d, disclosed by Dion et al, applies two pressure control mechanisms for limiting the reservoir underpressure. The first pressure control mechanism limits reservoir underpressure by introducing replacement fluid (i.e. air or ink) thereto. The second pressure control mechanism limits reservoir underpressure by changing the volume thereof. The two pressure control mechanisms cooperate to regulate the underpressure in the reservoir within a desired range. However, the mechanisms of Dion are rather complicated and occupy more space in the reservoir.
In contrast, a simpler mechanism, disclosed by Pollacek, et al in U.S. Pat. No. 5,040,002, xe2x80x9cRegulator for ink-jet pensxe2x80x9d, provides a regulator that comprises a seat and associated valve element. The seat is mounted to the body of an inkjet pen reservoir. The seat has a port formed through it. Magnetism is employed to attract the seat and valve element together and thereby close the port and permit underpressure to develop in the reservoir. When the underpressure within the reservoir rises above the level that may cause failure of the inkjet print head, the valve element moves away from the seat to permit air to enter the reservoir, thereby reducing the underpressure to an operable level. However, the magnetic mechanism is influenced when a strong magnetic force is to close to the inkjet pen, for example, during transportation, the underpressure is changed and the function and quality of printing may be influenced.
Another kind of underpressure regulator includes a flexible bag mounted to a flat curved spring. The elasticity of the spring tends to contract the bag as the bag expands in response to back pressure reduction in the reservoir. As disclosed in U.S. Pat. No. 5,409,134, xe2x80x9cPressure-sensitive accumulator for ink-jet pensxe2x80x9d by Cowger, et al, the flexible bag varies its volume between a minimum volume position and a maximum volume position to regulate the inkjet pen reservoir volume and adjust the underpressure so that the underpressure remains within an operating range that is suitable for preventing ink leakage while permitting the print head to continue ejecting ink drops. This kind of regulator, however, encounters the difficulty of exhausting the ink in the reservoir since the flexible bag has an expansion limitation. When the ink in the reservoir is low, the flexible bag has expanded to its limit, and the higher underpressure then causes the inkjet to fail and the rest of the ink cannot be used up. Furthermore, the ideal operative range of underpressure is within negative 2.5 to negative 10 cm water column, or xe2x88x920.0024 to xe2x88x920.0097 atmospheric pressure, which is so small that the elasticity of the spring has to be precisely controlled. The elasticity of the spring involves the technical problems of the contents of the material, the heat treatment process, and variations of shape, length and thickness of the spring, which cause instability of the spring characteristics. Consequently, Cowger, et al further discloses in U.S. Pat. No. 5,505,339, xe2x80x9cPressure-sensitive accumulator for ink-jet pensxe2x80x9d, some suitable shapes for the spring.
The primary object of the invention is to provide a reservoir mechanism for an inkjet pen like Pollacek""s, but one that is simpler and is not influenced by external magnetic force.
The inkjet pen according to the invention includes an ink reservoir for storing ink and providing ink for jetting. The reservoir includes a rigid body for storing ink, a port located on top of the rigid body, fluid-communicated with the ambient air for adjusting the air pressure inside the ink reservoir, and a valve operated by a spring or a resilient element for normally sealing the port but occasionally opening the port to introduce air into the reservoir when the ink level is low and the underpressure rises. In another embodiment, an elastic bag is included in the reservoir and has an opening communicated with the ambient air through a second port formed on top of the reservoir. The elastic bag expands in response to the increasing underpressure generated in the reservoir when ink is being used. The bag expansion actuates the opening of the valve so as to regulate the underpressure.